Flexible, tubular device e.g. a bellows

ABSTRACT

The present invention concerns a flexible, tubular device e.g. a bellows with an internal diameter up to 60 millimeters, said device comprising one or more corrugated convolutions ( 2 ), said convolutions having an overall bell-like shape with rounded top portions (T) and rounded bottom portions (B,B′). The novel aspects involve that the curvature of the outside surface of the convolutions ( 2 ) is numerically smaller at the top portions (T) than at the bottom portions (B,B′), said curvature being derived from a curve ( 6 ) defined as the intersection of the outside surface ( 4 ) of the device and a plane through the longitudinal axis ( 8 ) of the device, as well as they involve that the curvature of said curve changes sign only once at a change position (P,P′) located between a top portion (T) and an adjacent bottom portion (B,B″), and that the length of a first section ( 7 ) on the curve ( 6 ) is at least 10% longer than the length of a second section ( 9 ) on the curve, said first section ( 7 ) extending from one change position (P) to an adjacent change position (P′) via a top portion (T), and said second section ( 9 ) extending from one change position (P) to an adjacent change position (P′) via a bottom portion (B,B′). This provides an improved design with increased durability due to increased flexibility at lower stresses, compared to the prior art.

The invention relates to a flexible, tubular metal device e.g. a bellowswith an internal diameter up to 60 millimeters, said device comprisingone or more corrugated convolutions, said convolutions having an overallbell-like shape with rounded top portions and rounded bottom portions,where the curvature of the outside surface of the convolutions isnumerically smaller at the top portions than at the bottom portions,said curvature being derived from a curve defined as the intersection ofthe outside surface of the device and a plane through the longitudinalaxis of the device, and where the curvature of said curve changes signonly once at a change position located between a top portion and anadjacent bottom portion, and where the length of a first section on thecurve is at least 10% longer than the length of a second section on thecurve, said first section extending from one change position to anadjacent change position via a top portion, and said second sectionextending from one change position to an adjacent change position via abottom portion.

Flexible, tubular devices such as bellows with one or more convolutionsimpart a degree of flexibility in pipelines carrying gas, air, water,steam, petrochemicals or any other substance at varying temperatures andpressures. Turbines, pumps, compressors, heat exchangers, reactors andvalves are typical types of equipment where bellows can be used toabsorb relative movements between the equipment and the connectingpipelines. Unless some compensation for these dimensional changes isprovided, high stresses will be induced in the equipment or the pipingand might lead to system failure. The inherent flexibility enablesbellows to absorb movements in more than one direction and, thereby,leaves a greater degree of freedom in designing the layout of the pipingsystem, compared to using conventional devices such as bends and loops.

In general, a bellows can be applied to four basic movement modes (or acombination of these): axial, angular, lateral, and torsional. Thetorsional mode is, however, often unwanted because it destabilises theconvolution in a way that reduces its ability to absorb other modes. Thebellow durability depends greatly on the geometry, the materialproperties, the manufacturing processes during production, and theboundary and load conditions, e.g. an unrestrained pressurised bellowwill expand longitudinally, whereas an axially constrained bellow willrestrain pressure thrust from the system without changing itsdimensional length. The lifetime of each convolution depends, therefore,on a variety of factors but, in particular, on the ability to absorbmovements while having geometry that avoids local peak loads.

The convolution geometry is often based on sound engineering principlesand years of experience. This know-how is then used to design a bellowsthat may be required to withstand exposure to large variations inambient temperature and pressure, e.g. during equipment start-up,one-time stretching under assembly or numerous movements when inoperation. Designing a bellows for a piping system often requires,therefore, a thorough system response examination in order to avoidunfavourable conditions that can later lead to bellow failure.

Within the industry, it is commonly accepted that the leading cause offailure for bellows is due to fatigue or one-time damage duringinstallation. In both cases the bellow is stressed beyond acharacteristic threshold value, which leads to failure. A way to resolvethis failure type, and thereby increase product liability, is to reducethe stresses during deformation by providing improved convolutiongeometry. The bellow geometry depends on the number of convolutions, aswell as on the total length, skirt length, wall thickness and innerdiameter. The convolution geometry depends on convolution height, pitchand wall thickness. These are the common design variables, which can beadjusted for a specific application through empirically based safetyfactors and years of experience. Often the design is based onmodifications of a “U” span, “S” span, “V” span, or “Ω” span, from whereengineering design data and safety factors are generated. These shapesare “built-up” from simple geometric shapes (primitives) like straightlines and circle sectors, which are easy to draw, analyse and easilyprogrammable into a CNC interface before cutting metal for the formingtools. When such a convolution shape, combined with a material, which isoften stainless steel, and a manufacturing process, fulfils customerrequirements and expectations, the design may become the best practisewithin a specific area, even without being the best solution.

The susceptibility of the convolution to fatigue failure is, therefore,increased by geometric stress raisers that are more predominant thanwall thickness variations such as material thinning in the convolutionnose-tip area as a result of the forming process. For this reason, thebellow failure modes have resulted in the production of bellows inexpensive high-grade materials, e.g. stainless steel, which allow a poorconvolution design to offer an acceptable performance durability.

A bellow may be required to withstand a very large number of loadcycles, such as those from a running engine. The cyclic stress rangecontrols the overall fatigue life of the bellow and if the engine goesthrough several start-up and shut-down phases, the stress range willcontrol the cumulative fatigue life. In both cases the fatigue lifedepends on the total number of completed cycles and upon the mean stressand total stress range to which the bellow is subjected. With adecreased amount of stress, a bellows will withstand a greatly increasednumber of repetitions before failure, whereas at a higher stress level,failure will occur after a relatively fewer number of reversals.

When bellows need to be specially designed for a high cyclic life, theliterature is scarce in specifying characteristic design variables ordimensional ranges for an optimal convolution shape. In these cases, thebellow manufacturer must be advised of the expected number of cyclesand, based on empirical generated design data from historical successfuldesigns in connection with estimated material and manufacturingconstants, a safe convolution design might emerge. The literature is, inaddition, limited with regard to estimating fatigue data for calculatingthe lifetime of current standard spans, e.g. the data varies greatlywhen the pitch-height convolution ratio varies.

Guidance on design of bellows etc. may e.g. be found in “Standards ofthe Expansion Joint Manufacturers Association, Inc.”, 25 North Broadway,Tarrytown, N. Y. 10591.

U.S. Pat. No. 6,006,788 discloses a metal tube having helicalcorrugations. The helical design is used for continuously winding a wirearound the tube. The helical corrugations cause twisting of the tubewhen the tube is subjected to bending. In many automotive applications,fuel lines, brake lines etc. the ends of tubes/pipes are fixed, butvibrations and other movements will create repeated bending and therebytwist. However, the twist is locked due to the fixed ends wherebysignificant stresses are caused. Repeated stresses lead to metal fatigueand component failure.

The shape of the corrugations, as displayed in FIG. 4 of U.S. Pat. No.6,006,788, is a combination of smaller circular sections with radius rand larger circular sections with radius R. Calculations have, however,shown that significant stress-raisers are present fx by the transitionsbetween the circular sections. Although the disclosed shape may obtainhigh flexibility, the stress-raisers will decrease the live span underrepeated loading due to fatigue. An improved design is hence desirable.

One object of the present invention is to provide an improved design ofa flexible device such as a bellows, for obtaining increased life spanand/or obtaining the option of using alternative materials, compared tothe known designs. Another object is to provide the option of using lesscostly materials and/or lighter materials.

The novel and inventive aspects of the device according to the inventioninvolve that said convolutions are placed perpendicular to alongitudinal axis of the device and that the curve in said first andsecond sections is continuous and has non-constant curvature.

All geometrical transitions and bends result in stress raisers and adesign must be obtained where the magnitude of such unavoidable presentstress raisers is reduced. Compared to the standard U-shapedconvolution, the increased flexibility is achieved by making higherconvolutions, without significantly increasing the convolution width. Atthe same time, having a smaller curvature in the top portion, therebyreducing the stress raisers for this region, reduces the total stresslevel during deformation of the convolution. The design with a curvehaving a curvature, which changes sign only once at a change positionlocated between a top portion and an adjacent bottom portion, provides asmooth transition between the top and bottom portions. To minimize thestress level by the top portion, the length of the first section on thecurve is at least 10% longer than the length of the second section onthe curve, both sections extending from the locations where thecurvature changes sign. In this way the locations, where the curvaturechanges sign, are shifted towards the bottom portions and thus provide adesign with narrow bottom portions and wider, smooth top portions, whichsignificantly reduces the overall stress level. This provides animproved design of the flexible device whereby increased life span isobtained. Also, the improved design provides options to use alternativematerials, compared to the known designs, e.g. aluminium instead ofstainless steel. In this way, less costly materials and/or lightermaterials may also be used.

In another embodiment the length of a first section on the curve may beat least 50% longer than the length of a second section on the curve,said first section extending from one change position to an adjacentchange position via a top portion, and said second section extendingfrom one change position to an adjacent change position via a bottomportion. This gives further priority to the shape at the top portionsand further reduces the overall stress level.

In a further embodiment the curvature of the convolutions maynumerically be at least 20% smaller by the top portions than by thebottom portions, which results in a reduction of the overall stresslevel.

The pitch-height ratio(q) may in preferred embodiments be between 0.7and 1.0.

In yet a further embodiment the curve between a bottom portion and anadjacent bottom section may have one global maximum placed at the topportion and two global minima, said minima being placed by the bottomportions, and the curvature by the global maximum of the curve may havea local minimum. This provides an improved shape with significantlyredyced effect of the present stress raisers, and a high degree ofutilization of the material.

In a still further embodiment the curvature of the curve between a topposition and an adjacent bottom section has a local minimum. Thisprovides a smooth shape without abrupt geometrical changes.

In a preferred embodiment a section of the curve, corresponding to oneconvolution from one bottom portion to an adjacent bottom portion, issymmetric about an axis perpendicular to the longitudinal axis andthrough the global maximum within the top portion.

In another preferred embodiment the majority of the convolutions may besubstantially identical.

In a further preferred embodiment the device may be made of an extrudedmetal alloy pipe and the convolutions may be formed in a deep drawingprocess such as elastomeric forming or hydro forming.

In yet a further preferred embodiment the metal alloy may be stainlesssteel or an aluminium alloy.

The device may be used for flexible coupling of pipes or tubes in avehicle, e.g. a car, preferably for the coupling of pipes or tubes inthe air-condition system. It may have a number of other uses, such as ina charge air cooling system etc.

In the following the invention is described with reference to thedrawings, which display examples of embodiments of the invention.

FIG. 1 shows part of a cross section of a flexible, tubular device, saidcross section being taken along a plane through the longitudinal axis ofthe device.

FIG. 2 shows a curve defined as the intersection of the outside surfaceof a flexible, tubular device and a plane through the longitudinal axisof the device.

FIG. 3 shows a curve defined as the intersection of the outside surfaceof a flexible, tubular device, according to prior art, and a planethrough the longitudinal axis of the device.

FIG. 4-5 show curves each defined as the intersection of the outsidesurface of a flexible, tubular device according to the state of the artand a plane through the longitudinal axis of the device.

FIG. 1 displays a flexible, tubular device comprising a number ofconvolutions 2. The outside surface of the device is indicated byreference number 4. The pitch and the height of the convolutions areindicated by the references q and w. The longitudinal axis of the deviceis indicated by the reference number 8.

The height w is determined by the maximum material elongation beforenecking occurs. Necking is the phenomena that the wall thickness locallybecomes very thin, and thereby result in a risk of cracking of thematerial, during a deep drawing process, e.g. elastomeric forming.

FIG. 2 displays a curve 6 with rounded top portion T and rounded bottomportions B and B′. The top portion T is located by the global optimum ofthe curve 6. The bottom portions B,B′ are located by the global minimaof the curve 6. The curvature of the top portion T is smaller than thecurvature of the bottom portions B, B′. A line 10 is indicated where thecurvature is approx. zero/null. This is further indicated by the changepositions P and P′. The section 7 extends from the change position P viathe top portion T to the change position P′. The section 7 is longerthan a section 9 which extends from a change position P via a bottomportion B,B′ to a change position P′. The convolutions may be placedperpendicular to the longitudinal axis 8 of the device or they may beformed as a helix along the longitudinal axis. FIG. 3 displays a curve22 indicating a pure U-shape of the convolutions. The curve 22 isconstructed merely by primitives, straight lines 16 and circular arcs14. This design involves serious stress raisers at the locations 18 and20. This may simply be verified e.g. by a FE analysis using commerciallyavailable software such as e.g. Cosmos® or Ansys®.

FIG. 4 displays a curve 24 indicating a pure U-shape of theconvolutions. The measurements are used for reference tests, Ref A (FIG.4) and Ref B (FIG. 5), according to table 2 below. FIG. 5 displays acurve 26, also indicating a pure U-shape. The designs of FIG. 4 and 5are believed to be the industry state of the art and are provided by thecompany RANFLEX India Pvt. Ltd. of India. The design of Ref A is theresult of several manufacturing trials based on years of experience. Thedesign of Ref B is also based on years of experience in manufacturingbellows, but has not been manufactured yet, which means that the actualfinal design may change a few tenths of a millimetre ( 1/10 mm). Thiswill however not significantly change the result outlined in table 2.For informative reasons it must be underlined that manufacturing theshapes depicted in the examples as well as the shapes according to FIG.4 and 5 takes state of the art manufacturing tools and skills. It ise.g. necessary to make sure that an adequate amount of material isavailable in order to avoid over-stretching of the material, whichcauses cracking.

EXAMPLES

In the following, a number of examples of designs according to thepresent invention are shown. Table 1 displays the internal diameters,wall thicknesses and material for the examples. In each example thedesign of one convolution is given, as well as the correspondingcurvature relating to the design. The design and curvature are presentedas a series of points, which are to be understood as points on a smoothcurve through said points, said curve not being depicted. In eachexample the design of the convultions is presented by circular markswhereas the curvature is presented as triangular marks. The horizontalaxis displays the width of a convolution from one side to another,starting at a bottom portion B, see FIG. 2, and ending at a bottomportion B′ via one top portion T. The vertical axis on the left sidedepicts the height. Both height and width are in millimeters. thevertical axis on the right side depicts the curvature and the units are1/mm (inverse millimeters). TABLE 1 Internal diameter Wall thicknessExample [mm] [mm] Material 1 8 0.5 AA5454 2 8 0.5 AA5049 3 17 0.5 AA50494 17 0.5 AISI 316L 5 25.4 1.0 AA5049 6 38.1 1.0 AA5049 7 50.8 1.0 AA5049

It may be noted that although example 3 and 4 are similar in shape, theheight of the colution in example 4 is larger. This is due to thedifference in material properties between stainless steel and thealuminum alloy. Example q [mm] w [mm] $\frac{q}{2w}$ Flex [mm] 1 2.921.50 0.97 0.10 2 3.03 1.62 0.94 0.15 Ref A 5.30 3.65 0.73 0.10 3 6.103.72 0.82 0.32 4 5.91 4.61 0.64 0.73 5 8.83 4.96 0.89 0.64 6 12.34 6.780.91 0.97

TABLE 2 Ref B 12.90 8.60 0.75 0.52 7 15.56 8.84 0.88 1.35

The column in table 2 marked “Flex” displays the flexibility of oneconvolution, said flexibility is expressed as the total deflection whenone end at a bottom portion B (FIG. 2) is fixed and the other end atbottom portion B′ is displaced between a first situation, just beforeyielding takes place in compression to a second situation, just beforeyielding takes place in tension.

The displacement is parallel to the longitudinal axis 8, FIG. 2, of thedevice. In the first situation B′ is nearer B (compression) than in thesecond situation (tension). The flexibilities are found by FE analysistaking material and manufacturing (thinning etc.) parameters intoconsideration and represent close approximations to factual behavior ofmanufactured flexible devices.

Also, in table 2 two references, Ref A and Ref B, are displayed. For RefA the material and wall thickness are the same as in example 3, i.e. AA5049 and 0.5 millimeters, but the design is a pure U-shape correspondingto FIG. 4. An improvement from a flexibility of 0.10 millimeters to 0.32millimeters constitutes a 200% improvement, although q and w arerelatively equal. For Ref B the material and wall thickness are the sameas example 7, i.e. M 5049 and 1.0 millimeters, but the design is a pureU-shape corresponding to FIG. 5. An improvement from a flexibility of0.52 millimeters to 1.35 millimeters constitutes a near 200%improvement, although q and w also here are relatively equal.

In this document the term “radius of curvature” is to be understood asthe radius of a circle that touches a curve in a point on its concaveside and is of such a size that the circle and the curve have commontangents to both sides of said point.

The term “curvature” is to be understood as the inverse radius ofcurvature.

In practice, radius of curvature and curvature may be found for bothconcave and convex surfaces. Utilizing high-resolution optical measuringtechniques, objective data could be generated for determining the radiusof curvature and the curvature of surfaces on a manufacturedconvolution. The data may also be used to determine whether anyarbitrary convolution found on the market is in conflict with the scopeof protection. To date, such systems easily generate 12-bit resolutionscanning with a measuring point distance down to a hundredth of amillimeter. This data can then be used for further analysis withstandard CAD software or mathematical analysis software such asMathcad®.

The pitch-height ratio is defined as the pitch q divided by two timesthe height w, i.e. (q/2w).

It is to be understood that the invention as disclosed in thedescription and in the figures may be modified and changed and still bewithin the scope of the invention as claimed hereinafter.

1 A flexible, tubular metal device with an internal diameter up to 60millimeters, the device comprising one or more corrugated convolutionsthat define an outside surface of the device and are orientedperpendicular to a longitudinal axis of the device, each of theconvolutions having oppositely-disposed rounded top and bottom portions,the outside surface of the device having first and second sections withchange positions therebetween each of the first sections extending fromone of the change positions to another of the change positions via oneof the top portions each of the second sections extending from one ofthe change positions to another of the change positions via one of thebottom portions, the length of each of the first sections being at least10% longer than the length of each of the second sections the outsidesurface having a non-constant curvature derived from a curve that iscontinuous in the first and second sections and defined by theintersection of the outside surface and a plane through a longitudinalaxis of the device, the curvature of the outside surface beingnumerically smaller at the top portions than at the bottom portions, thecurvature of the curve changing sign only once at each of the changepositions. 2 A device according to claim 1, wherein the length of eachof the first sections is at least 50% longer than the length of each ofthe second sections. 3 A device according to claim 1, wherein curvatureof the convolutions is numerically at least 20% smaller within the topportions than within the bottom portions. 4 A device according to claim1, wherein the convolutions have a pitch-height ratio (g) of about 0.7to about 1.0. 5 A device according to claim 1, wherein the curve hasglobal maximums located at the top portions and has global minimumslocated at the bottom portions. 6 A device according to claim 1, whereinthe curve has a local minimum curvature between each adjacent pair ofthe top and bottom portions thereof. 7 A device according to claim 5,wherein a section of the curve extends from a point corresponding to theglobal minimum at a first of the bottom portions through the globalmaximum at an immediately adjacent one of the top portions and to apoint corresponding to the global minimum at a second of the bottomportions immediately adjacent the one of the top portions, the sectionof the curve being symmetric about an axis perpendicular to thelongitudinal axis and through the global maximum within the immediatelyadjacent one of the top portions. 8 A device according to claim 1,wherein a majority of the convolutions are substantially identical. 9 Adevice according to claim 1, wherein the device is made of an extrudedmetal alloy pipe and in that the convolutions are formed in a deepdrawing process. 10 A device according to claim 9, wherein the metalalloy is stainless steel or an aluminium alloy. 11 A flexible, tubularbellows with an internal diameter up to 60 millimeters, the bellowscomprising corrugated convolutions that define an outside surface of thebellows and are oriented perpendicular to a longitudinal axis of thebellows, each of the convolutions having oppositely-disposed rounded topand bottom portions, the outside surface of the bellows having first andsecond sections with change positions therebetween, each of the firstsections extending from one of the change positions to another of thechange positions via one of the top portions, each of the secondsections extending from one of the change positions to another of thechange positions via one of the bottom portions, the length of each ofthe first sections being at least 10% longer than the length of each ofthe second sections, the outside surface having a non-constant curvaturederived from a curve that is continuous in the first and second sectionsand defined by the intersection of the outside surface and a planethrough a longitudinal axis of the bellows, the curvature of the outsidesurface being numerically smaller at the top portions than at the bottomportions, the curvature of the curve being zero at the change positionsand changing sign only once between adjacent pairs of the top and bottomportions at the change position therebetween.